Metal forming method and apparatus

ABSTRACT

The specification and drawings disclose a method and apparatus for forming dish-shaped metal elements through the use of a combined fluid pressure and sheet tensioning process. The disclosed method is particularly intended for forming dish-shpaed sheet metal elements having a length substantially greater than their width and includes applying a longitudinal tension to the sheet while permitting the lateral edges to move toward one another substantially unconstrained while the fulid pressure acts to bow the sheet. The disclosed apparatus includes a flexible frame of generally eliptical shape which clamps to the edge of the sheet. The frame is arranged so that as the sheet bows outwardly, the lateral sides of the frame are pulled inwardly driving the ends of the frame apart.

United States Patent 1 Swenson, Sr.

[ July 3,1973

[ METAL FORMING METHOD AND APPARATUS [75] lnventor: Paul F. Swenson,Sr., Cleveland,

Ohio

[73] Assignee: Great Lakes Sports Mfg. Co.,

Cleveland, Ohio [22] Filed: Aug. 31, 1971 {21] Appl. No.: 176,519

[52] US. Cl 72/54, 29/421, 72/350, 9/6 [51] Int. Cl. B2ld 24/04 [58]Field of Search 72/54, 60, 62, 350, 72/351; 29/421; 113/120 R; 9/6, 6.5

156] I References Cited UNITED STATES PATENTS 1,763,582 6/1930 Gulick29/421 2,086,134 7/1937 vLudwick 29/421 3,164,894 1/1965 Johnson et al.29/421 1,970,134 8/1934 Ferris 72/351 3.458.917 8/1969 Mueller 29/421Primary Examiner-Richard .l. Herbst Attorney-Robert J. Fay et al.

[57] ABSTRACT shape which clamps to the edge of the sheet. The frame.

is arranged so that as the sheet bows outwardly, the lateral sides ofthe frame are pulled inwardly driving the ends of the frame apart.

14 Claims, 10 Drawing Figures METAL FORMING METHOD AND APPARATUS Thesubject invention is directed toward the art of metal forming and, moreparticularly, to an improved method and apparatus for stretch formingmetal sheet.

The invention is especially suited for forming boat hulls and will bedescribed with particular reference thereto; however, as will becomeapparent, the invention is capable of much broader application and couldbe employed for forming many different structures.

Typically, sheet metal elements and structures having compoundcurvatures must be formed by drawing operations. The die costs for suchoperations vary, of course, depending upon the size of the part, theamount of curvature, the depth of the draw, etc. Even for relativelysmall parts with simple, shallow draws, the die cost is substantial.However, when dealing with large, deep draw components such as boathulls, fuel tanks or the like, the die and press costs become extremelyhigh and can only be justified for very large production runs. Forrelatively small runs, other fabricating techniques involving much moreextensive use of manual labor will prove to be less costly to employ.

Techniques for forming sheet metal into hollow or dish-shaped structureswithout the use of cooperating dies have been proposed in the past.-Forexample, heat exchange plates have been made by bonding two sheets ofductile metal together about an area and then pressurizing the unbondedarea between the two sheets. The pressurizing caused the metal to bedeformed outwardly beyond its elastic limit. In this system, the metalwas constrained only by the surrounding welded metal and wrinkles andcreases would invariably develop in certain areas. Further, the depth ofthe draw was limited by the percent of elongation which the metal couldundergo without rupture. Although undesirable, these limitations weretolerable in some applications.

The subject invention provides a method by which large components of thetype mentioned can be fabricated rapidly and inexpensively with aminimum of tooling- The method and the apparatus allow fabrication ofsheet metal structures which could heretofore only be made by use ofdeep drawing dies and large, double action presses. Further, whenemployed as described herein, this method and apparatus permits thefabrication of the above-mentioned structures without the formation ofwrinkles and creases.

According to one aspect of the invention, there is provided a method offorming sheet metal into dishshaped structures having a length greaterthan their width. The method includes the steps of:

a. providing a metal sheet with a length nearly as great as the actuallength of the desired structure and a width substantially as great asthe width of the structure measured over its surface;

b. sealing the sheet to a fluid impervious surface continuously about anarea having a width substantially as great as the surface width of thedesired structure and a length slightly less than the actual length ofthe desired structure; and,

c. thereafter, creating a pressure differential across the sheet andwithin the sealed area sufficiently to deform the sheet outwardly awayfrom the surface while simultaneously applying a tension forcelongitudinally of the sheet.while permitting the lateral edges of thearea to move toward one another.

Preferably, and in accordance with a more limited aspect of theinvention, the air impervious surface is a second sheet of metal havingdimensions generally corresponding to the first sheet. Also, the sealingis preferably accomplished by clamping the two sheets together about thearea described.

Although the above described method could be carried out with a varietyof structures and apparatus, the invention also concerns a particularlysimple, flexible frame assembly which allows the required stretching orelongating forces to be generated by the pressurizing fluid. Broadly,the frame includes a pair -of generally elliptically-shaped clamp barssized so as to encircle the noted area and engage on opposite sides ofthe two sheets. Means, such as mechanical clamps, are provided fortightly clamping the two frame sections to the sheets to seal the spacebetween the sheets and within the area. Also, means are provided forsupplying fluid pressure within the frame and between the sheets. The

lateral sides of the frames are sufficiently flexible so that as thesheets are moved apart by the fluid pressure, the sides of the framesare pulled toward one another. Because the circumference of the frame isconstant, the ends of the frame must move outwardly or apart as thesides are pulled together. The outward movement of the ends of the frameautomatically applies longitudinal tension to the two sheets.

Within limits, the frame assembly can replace extremely expensiveforming dies and perform deep draw-type forming operations. The framecauses the resultant structure to have substantially no elongation ofthe metal in the transverse direction since the lateral sides of theframe merely move inwardly as the forming takes place. i

The invention also contemplates that the inward movement of the lateralsides can be limited at various points such as by fixed stops, toinfluencethe shape of the resulting structure.

Accordingly, a primary object of the invention is the provision of amethod and apparatus particularly suited for forming dish-shaped sheetmetal structures without the use of cooperating dies.

Yet another object is the provision of a forming method of the typedescribed wherein the metal is subjected to little or no elongation inits transverse dimension.

A still further object is the provision of an extremely simple, flexibleframe assembly which allows the method to be carried out veryinexpensively.

The above and other objects and advantages will become apparent from thefollowing description when read in conjunction with the accompanyingdrawings wherein:

FIGS. 1 through 4 are pictorial views somewhat diagrammatic showingthesequence of operations used for forming metal in accordance with apreferred embodiment of the invention; 7

FIG. 5 is a plan view, somewhat diagramatic, showing the changes whichtake place in the dimensional relationships of the metal sheet duringthe forming operation shown in the FIGS. 1 through 4 embodiment;

FIGS. 6 and 6a are cross-sectional views taken on lines 6-6 and 6a--6aof FIG; 5;

FIG. 7 is a plan view of an apparatus which can be used for carrying outthe methods illustrated in FIGS.

1 through 4;

sequence of steps could vary substantially and differ from that whichwill be shown and described; however, in the preferred form of theinvention, the forming process begins by the provision of two sheets ofmetal and 12, each having an extent at least sufficient to form theblank for the structural component desired. The size of the startingsheets or blanks relative to the finished component will be describedsubsequently.

In the embodiment under consideration, the finished component to beformed is generally eliptical having a dish-shaped configuration and alength substantially greater than its width. In the FIG. 1 showing, theminimum length of the blanks 10 and 12 are slightly less than the lengthof the finished component to be formed. Their minimum width ispreferably as great as the actual surface width of the finishedcomponent. To explain, assume that it is desired to form a dish-shapedmember having a length of approximately 16 feet and an actual widthmeasured over its surface of approximately 30 inches. To form acomponent of this general configuration by the subject method requiresthat the minimum length of the sheet preferably be, for example, in therange of 14 to 16 feet long and that its width be at least nearly 30inches wide.

In FIG. 1, a desired starting configuration for the structural componentwhich is to be formed is illustrated on the sheet 10 with the dottedline 14. The sheets 10 and 12 are illustrated as rectangular but theycould be other shapes so long as they are larger than the areaencompassed by line 14. Also, the sheets can be cut to the exact shapeand size of the line 14. This will be illustrated with respect to thepreferred form of apparatus shown in FIGS. 7 through 9.

Line 14 is empirically derived mathematically as a function of the finalshape desired for the component being formed. To form a hull of the typeunder consideration, it generally is eliptical and its total length L isonly slightly less than the resulting desired length of the hullstructure. Its width D is preferably substantially equal to the actualwidth of the hull measured over the surface. That is, D equals thesurface dimension of onehalf of the hull.

In the subject embodiment, the component to be formed is one-half of anelongated, hollow aluminum hull member which is intended for use as oneof the hulls of a catamaran. The complete hull is formed by dishing orforming two sheets of aluminum and joining them along mating edges. InFIG. 1, sheets 10 and 12 will be simultaneously formed to each formone-half of the hull structure.

In accordance with the invention, the sheets 10 and 12 are joinedtogether along line 14, preferably by clamping, so that they are in airtight sealed engagement along the line. FIG. 2 diagramaticallyillustrates clamping pressure applied to the sheets along line 14 by aseries of arrows 15.

With the two sheets 10 and 12 in sealed engagement along line 14, fluidpressure is introduced between the sheets and within the sealed area ofline 14. The actual pressure required for carrying out the invention isrelatively small and for the case of thin aluminum sheets of the sizereferred to, the pressure in the range of 5 to 10 psi will achieve thedesired results.

While the sheets are tightly sealed along line 14 and during the timethe fluid pressure is introduced, a substantial longitudinal tension issimultaneously applied as shown by the arrows 17 in FIG. 3. The tensiongenerated must preferably be sufficient to cause the sheets toplastically deform and to be elongated to the desired final length. Thetension required can be easily calculated from the width and thicknessof the sheets and known parameters of the metal being formed. Thepercent change in length due to plastic deformation will be at least 5percent when formed with the preferred apparatus but can, of course, beless or even substantially greater and up to the ultimate permissiblefor the metal.

During the inflation and longitudinal tensioning, the lateral edges ofthe sheets are constrained only slightly, or not at all and arepermitted to draw toward one another as the metal deforms outwardly.They are permitted to move beyond the desired final spacing, i.e. thewidth of the structure, by an amount necessary to accommodate the springback, inherent in forming metals. Further, the edge of the area, i.e.line 14, is preferably held in a single plane and is not permitted tobend or twist.

FIG. 4 shows the two sheets at the completion of the stretching andinflating process. Note that the sheet 10 has been dished outwardlywithin the area of line 14. (Although not shown, sheet 12 is similarlydished.) Additionally, the sheets have been elongated in thelongitudinal direction. As mentioned, during the inflating process, thelateral edges are restrained or constrained only slightly so that thetransverse dimension is not stretched but permitted to move inwardly asthe inflation takes place. Thus, the metal in the resulting dishshapedstructure has been strained and elongated in the longitudinal dimensionbut substantially unstrained in the transverse dimension.

FIG. 5 illustrates the plan view of one sheet within the line 14 beforeand after the forming process. The solid line shows the line 14 at thecompletion of the forming process. As best shown in FIGS. 6 and 6a,little or no elongation takes place in the sheet transversely to itslongitudinal direction. However, in the longitudinal direction, thelength of the sheet has been increased substantially. The amount oflongitudinal stretch which the sheet can undergo is, of course,determined by the particular type of material being formed. For example,with aluminum, elongation of 12 percent to 25 percent can be achievedwithout tearing of the sheet, depending on the particular alloyemployed.

Many different shapes have been formed by use of the described method.The best results have been achieved, however, when there are no sharpcorners, re-entrant angles or curves and when the length is at leasttwice the width. Further, although it is preferred to not strain thesheet in the transverse dimension, good results can be obtained withsome straining.

Many different types of structures and apparatus could be used forcarrying out the described method. For example, the clamping or sealingcould be accomplished by pneumatically or hydraulically actuatedclamping jaws and the elongation can similarly be done through the useof fluid cylinders or mechanically. One

aspect of the subject invention, however, concerns the provision of anextremely simple, flexible frame assembly which permits the internalpressures acting within the sheets to produce the forces required forsheet elongation. FIGS. 7 through 9 show a preferred form of highlysimplified apparatus in which this particular technique can be carriedout.

Referring in particular to FIG. 7, the apparatus is shown as comprisinga relatively flexible frame assembly 20 having an eliptical contourcorresponding to the starting configuration of line 14 of FIGS. 1through S.v

The frame assembly 20 includes a pair of frame members 22 and 24 which,in the subject embodiment, are formed from steel bar bent to theeliptical shape illustrated and joined at their ends. As will becomeapparent, the required strength of the bars will vary depending upon theparticular structural configuration being formed and the longitudinalcompressive loads to which the bars will be subjected during a formingoperation. In the embodiment shown, the bars have 1 a inch X 1% inchcross-section throughout their entire circumference. The bars are eachof identical shape and are arranged to engage and grip the sheets and 12along the line 14.

Many different types of gripping arrangements could be provided so thatthe sheets are tightly held and sealed along line 14. FIG. 9 shows asimple form of sealing and gripping jaw used in the subject embodiment.Note that bar or frame member 22 has a small groove 26 formedcontinuously about its lower edge surface. Bar 24 has a similar, alignedgroove 28 formed about its top edge surface and a metal key or tonguemember 30 is positioned within the groove to extend outwardly a shortdistance. Thus, when the bars are clamped on the sheets 10 and 12, thesheets are sealed and tightly gripped by cooperation between theoutwardly extending portion of strip 30 and the groove 26.

The required clamping force can be applied to the two bars 22 and 24 inmany different ways. For example, air or hydraulic cylinders can be usedto draw the clamp bars together. In the embodiment under consideration,the clamping is accomplished by a large number of simple clampassemblies 32. The clamp assemblies 32 are best shown in FIG. '8 andeach comprise a pair of L-shaped bar members 34 which engage the clampbars 22 and 24 in the manner shown. A bolt 36 extends through alignedopenings in the members 34. By tightening the nuts 38, the frameassemblies can be tightly clamped together. It is important to note thatthe clamp assemblies 32 are arranged so that they do not affect theflexibility of the frame assembly. That is,

the lateral sides of the frame can flex inwardly during other of sheets10 and 12, or by a small fitting extend ing inwardly between the twosheets from the edge. During application of fluid pressure to the spacebetween the sheets, the sheets tend to bow outwardly, i.e. toward andaway from the viewer of FIG. 7. During this outward bowing of thesheets, the lateral sides of the frames are pulled inwardly toward oneanother because of the geometry of theframe and its flexibility.Simultaneously with the inward movement of the lateral sides,

the end portions of the frame are driven outwardly. That is, the lengthof the sheets 10 and 12 must increase because the circumference of theframe remains constant while its width decreases. Thus, the frameautomatically applies the required longitudinal stress while permittingthe lateral edges to move inwardly substantially unconstrained.

Depending upon the final shape desired, one or the other or both of thelateral sides can be constrained at various points during the inflationprocess to influence the final shape of the structure. In the subjectembodiment, it is desirable that the edge 40 of the structures have asomewhat straight configuration. For this reason, the frame assemblyincludes a rigid beam 42 in the form of a steel channel. The frame 42 isconnected to the bar 22 by a plurality of tie rods 44. The tie rods 44have nuts 46 adjusted so that as the lateral sides of the frame sections22 and 24 move inwardly, the nuts engage the beam at various pointsproviding a stop to prevent further inward movement of the edge. Throughthe use of this arrangement, the amount of flexure which either one orboth of the sides undergo can be varied. It should be appreciated thatvarious other types of frame assemblies and motion limiting arrangementscould be used for carrying out the invention. Additionally, although thelength of the structures formed must be greater than their width topermit use of the inventive method, the variationscan be substantial.

The invention has been described in great detail sufficient to enableone of ordinary skill in the metal forming art to make and use the same.Obviously, modifications and alterations of the preferred embodimentwill occur to othersupon a reading and understanding of thespecification and it is my intention to include all such modificationsand alterations as part of my invention insofar as they come within thescope of the appended claims.

What is claimed is: l

1. A method of forming a sheet of metal into a dishshaped structurehaving a length substantially greater than its width comprising thesteps of:

a. providing a metal sheet having a length at least nearly as great asthe actual length of the desired structure and a width which is at leastnearly as great as the width over the surface of the desired structure;b. providing an air impervious surface; 0. sealing said sheet to saidair impervious'surface continuously about an area having a widthsubstantially as great as the width over the surface of the desiredstructure and a length less than the length over the surface of saiddesired structure; fluid pressurizing the space between said sheet andsaid surface within said sealed area sufficiently to cause said sheet todeform outwardly away from said surface while simultaneously permittingthe lateral edges of said area to move inwardly so asto producesubstantially no strain on said sheet in the direction of its width andsimultaneously therewith applying a longitudinal tension force to saidsheet to strain it sufficiently to elongate said sheet to the desiredlength of said structure.

2. The method as defined in claim 1 including-the step of maintainingthe-lateral margins of said area sealed while permittingthem to moveinwardly to the actual width of said desired structure.

3. The method as defined in claim 1 wherein said surface is defined by asecond sheet of metal.

4. The method as defined in claim 1 wherein the perimeter of said areais maintained constant throughout said pressurizing.

5. The method as defined in claim 1 wherein said surface is defined by asecond sheet of metal and wherein said two sheets are joined about saidsealed area.

6. The method as defined in claim 5 wherein said sheets are joined aboutsaid sealed area after being pressurized.

7. The method as defined in claim 5 wherein said sheets aresimultaneously elongated to the actual length of said desired structure.

8. The method as defined in claim 5 wherein at least a portion of onelateral edge of said area is prevented from moving during at least aportion of the time said pressurization takes place.

9. A method of forming a dish-shaped sheet metal structure having anactual length substantially greater than its actual width comprising thesteps of:

a. providing a sheet of metal having a width at least nearly as great asthe width over the surface of said desired structure and a length atleast nearly as great as the surface length of said desired structure;

b. gripping said sheet continuously about an area having a widthsubstantially equal to the surface width of said desired structure and alength slightly less than the actual length of said desired structure;

0. applying pressure uniformly over said area to deform said sheet whilesimultaneously applying a tension force to the ends of said area tostrain said sheet and elongate said area to the actual length of saiddesired structure; and,

d. during application of said pressure, constraining the lateral sidesof said area to move toward one another without appreciably strainingsaid sheet across its width as said sheet is deformed until said lateralsides reach the actual width of said desired structure.

10. A method of forming a dish-shaped sheet metal structure comprisingthe steps of:

a. gripping a sheet of metal continuously along a line about an areahaving a length substantially greater than its width; pl b. applyings'ufficient pressure uniformly to said area to deform said sheetoutwardly while maintaining said line in its original plane;

c. simultaneously applying a tension force with the application of saidpressure applying force to the ends of said area to elongate the sheet;and,

d. constraining the lateral edges of said sheet in a manner to avoidsubstantial stretching of said sheet across the width of said area.

11. The method as defined in claim 10 wherein said area has a length atleast twice as great as its width.

12. The method as defined in claim 10 wherein said force applied to theends of said area is applied continuously during application of saidpressure.

13. The method as defined in claim 12 wherein said force varies inrelation to said pressure.

14. The method as defined in claim 10 wherein said force applied to theends of said area is proportional to the pressure applied to said area.

1. A method of forming a sheet of metal into a dish-shaped structurehaving a length substantially greater than its width comprising thesteps of: a. providing a metal sheet having a length at least nearly asgreat as the actual length of the desired structure and a width which isat least nearly as great as the width over the surface of the desiredstructure; b. providing an air impervious surface; c. sealing said sheetto said air impervious surface continuously about an area having a widthsubstantially as great as the width over the surface of the desiredstructure and a length less than the length over the surface of saiddesired structure; d. fluid pressurizing the space between said sheetand said surface within said sealed area sufficiently to cause saidsheet to deform outwardly away from said surface while simultaneouslypermitting the lateral edges of said area to move inwardly so as toproduce substantially no strain on said sheet in the direction of itswidth and simultaneously therewith applying a longitudinal tension forceto said sheet to strain it sufficiently to elongate said sheet to thedesired length of said structure.
 2. The method as defined in claim 1including the step of maintaining the lateral margins of said areasealed while permitting them to move inwardly to the actual width ofsaid desired structure.
 3. The method as defined in claim 1 wherein saidsurface is defined by a second sheet of metal.
 4. The method as definedin claim 1 wherein the perimeter of said area is maintained constantthroughout said pressurizing.
 5. The method as defined in claim 1wherein said surface is defined by a second sheet of metal and whereinsaid two sheets are joined about said sealed area.
 6. The method asdefined in claim 5 wherein said sheets are joined about said sealed areaafter being pressurized.
 7. The method as defined in claim 5 whereinsaid sheets are simultaneously elongated to the actual length of saiddesired structure.
 8. The method as defined in claim 5 wherein at leasta portion of one lateral edge of said area is prevented from movingduring at least a portion of the time said pressurization takes place.9. A method of forming a dish-shaped sheet metal structure having anactual length substantially greater than its actual width comprising thesteps of: a. providing a sheet of metal having a width at least nearlyas great as the width over the surface of said desired structure and alength at least nearly as great as the surface length of said desiredstructure; b. gripping said sheet continuously about an area having awidth substantially equal to the surface width of said desired structureand a length slightly less than the actual length of said desiredstructure; c. applying pressure uniformly over said area to deform saidsheet while simultaneously applying a tension force to the ends of saidarea to strain said sheet and elongate said area to the actual length ofsaid desired structure; and, d. during application of said pressure,constraining the lateral sides of said area to move toward one anotherwithout appreciably straining said sheet across its width as said sheetis deformed until said lateral sides reach the actual width of saiddesired structure.
 10. A method of forming a dish-shaped sheet metalstructure comprising the steps of: a. gripping a sheet of metalcontinuously along a line about an arEa having a length substantiallygreater than its width; b. applying sufficient pressure uniformly tosaid area to deform said sheet outwardly while maintaining said line inits original plane; c. simultaneously applying a tension force with theapplication of said pressure applying force to the ends of said area toelongate the sheet; and, d. constraining the lateral edges of said sheetin a manner to avoid substantial stretching of said sheet across thewidth of said area.
 11. The method as defined in claim 10 wherein saidarea has a length at least twice as great as its width.
 12. The methodas defined in claim 10 wherein said force applied to the ends of saidarea is applied continuously during application of said pressure. 13.The method as defined in claim 12 wherein said force varies in relationto said pressure.
 14. The method as defined in claim 10 wherein saidforce applied to the ends of said area is proportional to the pressureapplied to said area.